1. Field of the Invention
This invention relates to devices and methods for bonding of semiconductor wafers.
2. Description of Related Art
Fusion bonding can bond two semiconductor wafers together to form sealed cavities between the wafers. U.S. Pat. Ser. No. 4,883,215 to Goesele et al., which is hereby incorporated by reference in its entirety, describes a known fusion bonding process such as illustrated in FIG. 1A. The bonding process bonds a first wafer 110 having a surface with a depression to a second wafer 120 to form a sealed cavity 115. In addition to the depression, wafer 110 may include semiconductor elements constructed in and on its surface before the fusion bonding. Wafer 120 is a uniform structure such as an unprocessed wafer so that alignment of wafers 110 and 120 is not critical. Alternatively, structures in and on wafer 120 are aligned with structures in and on wafer 110 in an aligned fusion bonding process such as described in U.S. Pat. Ser. No. 5,273,118 to Bower et al., which hereby is incorporated by reference herein in its entirety.
A conventional two-step fusion bonding of wafers 110 and 120 takes place in an ambient that is about 90% or more oxygen at atmospheric pressure. A first step of the process places cleaned smooth surfaces of wafers 110 and 120 in contact to form an initial bond in a bond area 112. The initial bond is due to water-hydrogen bonding and seals the oxygen rich gas in cavity 115. A second step is annealing at an elevated temperature, about 1000 .degree. C., to form a strong chemical bond between wafers 110 and 120. During annealing, oxidation of the interior surfaces of cavity 115 consumes the oxygen in cavity 115.
During the bonding process, the bonding surfaces of the two wafers must be kept in contact. Otherwise, bubbles can form between bonding surfaces in area 112, and cavity 115 may not seal. For wafers 110 and 120 to stay together until the bonding process is complete, forces on wafers 110 and 120 must satisfy Equation 1 throughout the bonding process. EQU FA+FB&gt;FG Equation 1
In Equation 1, FA is the force of the exterior or ambient pressure pressing wafers 110 and 120 together; FB is the bonding force in area 112 which holds wafers 110 and 120 together; and FG is the outward force of the gas trapped in cavity 115. For Equation 1 to hold, the combination of the bonding force FB and the force of the ambient FA must be larger than the force FG of the trapped gas. Increasing the size of bonding area 112 increases force FB but also increases device area and costs. Force FG is proportional to the total number of gas molecules in cavity 115 and increases linearly with temperature. The number of gas molecules in cavity 115 depends on the ambient surrounding wafers 110 and 120 during the initial wafer bonding. During annealing, the elevated temperature increases force FG until oxidation of the interior surfaces of cavity 115 consumes oxygen molecules and reduces the pressure in cavity 115 and force FG. Ideally, a vacuum forms in cavity 115 when all the oxygen and any other gases in cavity 115 are consumed. Wafer 120 can then be thinned and shaped to form semiconductor devices such as described in U.S. Pat. No. 5,576,251, to Garabedian et al., which hereby is incorporated by reference herein in its entirety.
Although known fusion bonding processes are suitable for manufacturing of such devices, processes and tools that simplify fusion bonding, reduce the required bonding area, and are inexpensive to implement are still desired.